With the development of technology and the increase of demand, people have increasingly stronger demands for an optical network with higher speed and larger bandwidth. However, in the circumstance of small channel spacing with high rate, since an optical pulse modulation technology based on the No Return Zero (NRZ) modulation format widely used at present has adverse factors such as comparatively serious nonlinear effect and crosstalk and so on, operational requirements can not be satisfied yet. Therefore, a non-NRZ optical pulse modulation technology has become a research hotspot of optical transmission technology in recent years.
Compared with a traditional NRZ modulation technology, the non-NRZ optical pulse modulation technology has the concentrated optical spectral energy and high spectrum efficiency, which can improve chromatic dispersion tolerance and nonlinearity tolerance in transmission with long distance and large capacity, and enhance the transmission performance of a DWDM system significantly.
In the entire DWDM system, modulation and demodulation of optical carriers are completed by an optical transponder. With regard to a system with a single channel at 40 Gb/s or above, in order to implement long distance transmission, optical transponders all use the non-NRZ optical pulse modulation technology basically, and in the non-NRZ modulation technology, the phase modulation technology obtains the most comprehensive recognition. However, the ultra-high rate raises more severe requirements on signal quality of the optical transponder, besides regular indexes such as bit error ratio and Optical Signal to Noise Ratio (OSNR) and so on, a jitter index is also an system performance index of great importance.
In a digital communication system, jitter describes the short-term relative change of a digital signal deviating from the ideal time and location at an important time point. The jitter has numerous different random sources and nonrandom (determinacy) sources. Thermal noise, craft, crosstalk and Electromagnetic Compatibility (EMI) radiation and so on may all cause the jitter. In the DWDM system, the jitter will influence the synchronization of subsystem, and excessive jitter will directly produce a big bit error, which seriously deteriorates the transmission performance of DWDM system. Therefore, with regard to the optical transponder as a core component of the DWDM system, it is necessary to inspect the jitter index of the optical transponder.
Currently, in a jitter meter, an optical interface in a special modulation code pattern is not considered, thus the jitter performance of an optical module with non-NRZ modulation can not be tested directly.